Speed control structure and method for work vehicle

ABSTRACT

A load control structure for a work vehicle comprises: set rotation speed detection device for detecting a set rotation of an engine of the work vehicle; actual rotation speed detection device that senses an actual rotational speed of the engine; continuously variable speed change device that receives power from an engine of the work vehicle; speed change position detecting device for detecting a speed change operation position of the continuously variable speed change device; operating device for speed-shifting the continuously variable speed change device; control device for controlling the operation of the operating device; wherein the control device calculates a drop amount of the actual engine rotation speed from the set rotation speed based on the detected information from the set rotation speed detection device and the actual rotation speed detection device, and sets a limit operation position for the continuously variable speed change device based on the calculated drop amount and a correlation data that correlates the actual engine rotation speeds with operating positions of the continuously variable speed change device, and controls the operating device such that the operating position of the continuously variable speed change device moves to the limit operating position based on the set limit operating position and detected information from the speed change position detecting device, and wherein the control device has command device for commanding a change of the correlation data.

BACKGROUND OF THE INVENTION

The present invention is directed to a load control structure and thecontrol method for a work vehicle.

Such a work vehicle conventionally has, among other things, set rotationspeed detection means for detecting a set rotation of an engine of thework vehicle; actual rotation speed detection means that senses anactual rotational speed of the engine; continuously variable speedchange device that receives power from an engine of the work vehicle;speed change position detecting means for detecting a speed changeoperation position of the continuously variable speed change device;operating means for speed-shifting the continuously variable speedchange device; control means for controlling the operation of theoperating means.

In the load control mechanism mentioned above, the control meanscontrols the control means for controlling the continuously variablespeed change device based on the drop amount of the engine rotationspeed from a set rotation speed so as to operate the continuouslyvariable speed change device to a speed position in accordance with thedrop amount. (See, for example, JP2002-22006.)

In the above structure, the continuously variable speed change device ismerely operated to a speed position in accordance with the drop amountfrom a set engine rotation speed. Thus, it was difficult to have asatisfactory load control that is good for different engine loads thatdiffer depending on operating conditions.

SUMMARY OF THE INVENTION

An object of the present invention is to provide advantageous loadcontrol for different engine loads that differ depending on differentoperating conditions.

Accordingly, a load control structure for a work vehicle in accordancewith the present invention comprises: set rotation speed detection meansfor detecting a set rotation of an engine of the work vehicle; actualrotation speed detection means that senses an actual rotational speed ofthe engine; continuously variable speed change device that receivespower from an engine of the work vehicle; speed change positiondetecting means for detecting a speed change operation position of thecontinuously variable speed change device; operating means forspeed-shifting the continuously variable speed change device; controlmeans for controlling the operation of the operating means; wherein thecontrol means calculates a drop amount of the actual engine rotationspeed from the set rotation speed based on the detected information fromthe set rotation speed detection means and the actual rotation speeddetection means, and sets a limit operation position for thecontinuously variable speed change device based on the calculated dropamount and a correlation data that correlates the actual engine rotationspeeds with operating positions of the continuously variable speedchange device, and controls the operating means such that the operatingposition of the continuously variable speed change device moves to thelimit operating position based on the set limit operating position anddetected information from the speed change position detecting means, andwherein the control means has command means for commanding a change ofthe correlation data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a tractor as an example of a work vehicle,

FIG. 2 is a schematic diagram showing the drive train of the tractor,

FIG. 3 is a sectional side view showing a part of the drive train of thetractor,

FIG. 4 is a sectional plan view showing a part of the drive train of thetractor,

FIG. 5 is a diagram showing the hydraulic circuit,

FIG. 6 is a block diagram showing the control structure,

FIG. 7 is a graph showing a correlation between the operated positionsof the speed change pedal and the operated positions of the pump swashplate,

FIG. 8 is a graph showing a correlation between deviations of the swashplate and target operation speeds,

FIG. 9 is a graph showing a correlation between positions of the swashplate and engine rotation rates,

FIG. 10 is graph showing the movement of the pump swash plate when themotor swash plate position is being changed.

FIG. 11 shows how a pump swash plate position is displayed on a LCD,

FIG. 12 is a sectional plan view showing a mechanical servo controlmechanism,

FIG. 13 is a hydraulic circuit diagram showing the mechanical servocontrol mechanism and the switch over mechanism;

FIG. 14 is a hydraulic circuit diagram showing an arrangement where onlyan electronic servo control mechanism is used;

FIG. 15 is a hydraulic circuit diagram showing an arrangement where onlya mechanical servo control mechanism is used;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The entire side view of the tractor as an example of a work vehicle isshown in FIG. 1. This tractor has a front frame 2 that supports theengine 1 via a vibration insulator, front wheels 3 supported to eitherside of the front frame 2, a transmission case 4 that also functions asa frame connected with the engine 1, and the rear wheels 5 provided toeither side of the transmission case 4. The work vehicle has theoperator's section 8 equipped with a steering wheel 6, the operator'sseat 7, etc. above a transmission case 4. The tractor has severalsensors as described below. These sensors are known and conventionalsuch as rotation sensors, which can be of optical or magnetic type orotherwise, and will not be described in detail below.

As shown in FIGS. 2-4, the power from the engine 1 is transmitted to thehydrostatic continuously variable speed change device (an example of acontinuously variable speed change device) 10 which functions as a mainspeed change apparatus through the dry-type main clutch 9 etc. Thedriving power from the hydrostatic continuously variable speed changedevice 10 is transmitted to the right and left front wheels 3 and theright and left rear wheels 5 through the gear type speed change device(an example of a stepwise change speed device) 11 which functions as anauxiliary speed change device that can be shifted to three speedpositions, high, medium, and low speed positions, the differential gear12 for front wheels, or the differential gear 13 for rear wheels. Thepower for an operation taken from the hydrostatic type continuouslyvariable speed change device 10 is transmitted to power take off axis 15via hydraulic operation clutch 14 etc.

Transmission case 4 is formed by connecting the four casing parts: thefirst casing part 4A that houses the main clutch 9 etc., the secondcasing part 4B that houses hydrostatic type continuously variable speedchange device 10 etc., the third casing part 4C that houses operationclutch 14 etc., and the fourth casing part 4D that houses gear typespeed change device 11 etc.

As shown in FIGS. 2-5, hydrostatic type continuously variable speedchange device 10 has a axial plunger type variable displacement pump 16,axial plunger type variable displacement motor 17, etc. which are housedin the second casing part 4B, where the power from variable displacementpump 16, which is not speed-changed, is outputted as power for anoperation and the speed-changed power from variable displacement motor17 is outputted as power for traveling. The charge oil from charge pump21 driven by the engine power is supplied to closed circuit 20 viacharge oil path 22, check valve 23, etc. which is formed by connectingthe variable displacement pump 16 and the variable displacement motor 17with the first oil path 18 and the second oil path 19.

As shown in FIGS. 1, and 4-6, this tractor is equipped, in itsoperator's station 8, with a servo control mechanism 25 with which swashplate (referred to as a pump swash plate hereinafter) 16A of variabledisplacement pump 16 is operated, based on operation of the speed changepedal (an example of speed change operating element) 24 which is urgedtoward its neutral position.

As shown in FIGS. 4-6, the servo control mechanism 25 has hydraulic pumpcylinder 26 for continuously operating the pump swash plate 16A (anexample of a operating means), a servo valve 27 which regulates flow ofthe hydraulic fluid to the hydraulic pump cylinder 26, a regulator valve28 that maintains the hydraulic pressure to the serve valve 27 at apredetermined value, a pedal sensor (an example of an operation positiondetecting means) 29 which has a potentiometer to detect the operatedposition of the speed change pedal 24, and a swash plate sensor (anexample of a speed change position detecting means) 30 which has apotentiometer which detects the operation position of pump swash plate16A from the amount of operation of the pump cylinder 26, and a controldevice (an example of a control means) 31 which has a microcomputer towhich detected information from the sensors such as the pedal sensor 29,the swash plate sensor 30, etc. are inputted.

The pump cylinder 26 is housed in the second casing part 4B with aforward travel decelerating spring 32 and reverse travel deceleratingspring 33 that urge the swash plate 16A toward its neutral position. Ashydraulic fluid is supplied to hydraulic chamber 34 for forward travelgear change, the pump swash plate 16A is operated to a forward travelspeed-increase (or accelerating) direction against the urging force ofthe forward travel decelerating spring 32. As hydraulic fluid issupplied to the hydraulic chamber 35 for reverse travel speed changes,pump swash plate 16A is operated to a reverse travel speed-increasedirection against the travel decelerating spring 33.

The servo valve 27 has an electromagnetic proportional valve 36 forforward travel to control a flow of hydraulic fluid to the hydraulicchamber 34 for forward travel of the pump cylinder 26 and anelectromagnetic proportional valve 37 for reverse travel which controlsa flow of the hydraulic fluid to the hydraulic chamber 35 for thereverse travel speed changes of the pump cylinder 26. The regulatorvalve 28 distributes the hydraulic fluid fed from supply pump 38 forpower steering to operation clutch 14 and hydraulic power steeringdevice 39 with pressure suitable for each operation. The supply oil path41 to servo valve 27 is connected to the pressure port 28A of theregulator valve 28 to which the supply oil path 40 to operation clutch14 is connected.

Control device 31 has at least a MPU, memory, and other known hardwarerequired to perform communication function, and other functions andalgorithms described in the specification. As shown in FIG. 6, thecontrol device 31 has the map data (an example of correlation data)which correlates the operated position (or actuation position) of speedchange pedal 24, with the operation position of pump swash plate 16A,and pump swash plate control means 31A which has the control programwhich operates pump swash plate 16A by controlling the operation ofproportional valve 36 for forward travel, or proportional valve 37 forreverse travel based on the map data and detected information or signalsfrom the pedal sensor 29, detected information from the swash platesensor 30, etc.

The map data of pump swash plate control means 31A makes thecorrespondence between the operated position of speed change pedal 24and the operation position of pump swash plate 16A such that the greaterthe amount of operation from the neutral position of speed change pedal24 to a forward travel speed-increase direction is, the greater theamount of operation from the neutral position of pump swash plate 16A toa forward travel speed-increase direction is, and such that the greaterthe amount of operation from the neutral position of speed change pedal24 to a reverse travel speed-increase direction is, the greater theamount of operation from the neutral position of pump swash plate 16A toa reverse travel speed-increase direction is (See FIG. 7).

The control program of pump swash plate control means 31A sets theoperation position of pump swash plate 16A corresponding to the operatedposition of speed change pedal 24 which pedal sensor 29 detected, as thetarget operation position of pump swash plate 16A based on the storedmap data and the detected information from pedal sensor 29.

Based on the set target operation position and the detected informationfrom swash plate sensor 30, the operation of the proportional valve 36for forward travel and the proportional valve 37 for reverse travel iscontrolled such that the target operation position of pump swash plate16A and the actual operation position come into agreement. By thiscontrol operation, the vehicle can be moved forward or reversed at thespeed in accordance with the operated position of speed change pedal 24.

That is, the servo control mechanism 25 is an electronically controlledtype where the pump swash plate control means 31A controls the operationof proportional valve 36 for forward travel, or proportional valve 37for reverse travel to operate the pump cylinder 26 in order to operatethe pump swash plate 16A of the hydrostatic type continuously variablespeed change device 10 based on detected information from pedal sensor29, and detected information from swash plate sensor 30. The servocontrol mechanism 25 drives pump cylinder 26 directly with the outputpressure of the proportional valve 36 for forward travel or proportionalvalve 37 for reverse travel which goes through pressure port 28A ofregulator valve 28 (direct-acting type).

A stable servo pilot pressure can be obtained and operation control ofpump cylinder 26 can be performed with sufficient accuracy, comparedwith an arrangement where the pump cylinder 26 is driven with the outputpressure from charge oil path 22 where pressure changes due to thepressure fluctuation in closed circuit 20 of hydrostatic typecontinuously variable speed change device 10, or changes in enginerotation rate. As a result, vehicle speed control which effects forwardtravel or reverse travel at speed in accordance with the operatedposition of speed change pedal 24 with sufficient accuracy based ondetected information from pedal sensor 29, and detected information fromswash plate sensor 30 while utilizing a direct-acting type servo controlmechanism 25, which is relatively inexpensive.

The control device 31 has; an operation program which computes thedeviation between the target operation position of the pump swash plate16A determined by the pump swash plate control means 31A and the actualoperation position based on the target operation position of pump swashplate 16A and the detected information from swash plate sensor 30; aplurality of map data (an example of correlation data) which correlatesthe deviation of the target operation position of pump swash plate 16Awith the actual operation position and the operating speed of pump swashplate 16A; and the first operating speed setting means 31B has thecontrol program which sets the target operating speed of pump swashplate 16A based on those map data and calculated result of the operationprogram.

Each map data of the first operating speed setting means 31B correlatesthe deviation of pump swash plate 16A with the operating speed of pumpswash plate 16A (refer to FIG. 8) such that when there is a largedeviation between the actual operation position of pump swash plate 16Adetected by swash plate sensor 30 and the target operation position ofpump swash plate 16A determined by the pump swash plate control means31A, the operating speed of the pump swash plate 16A becomes greater,and such that operating speed of pump swash plate 16A for a givendeviation of the pump swash plate 16A in reverse travel is less than theoperating speed of pump swash plate 16A for a given deviation of pumpswash plate 16A in a forward travel.

The control program of the first operating speed setting means 31B isset up such that the operating speed of pump swash plate 16Acorresponding to the deviation of computed pump swash plate 16A is setas the target operating speed of pump swash plate 16A based on the mapdata and the calculation result of the operation program. And the settarget operating speed is output to the pump swash plate control means31A.

In vehicle speed control, the control program of the pump swash platecontrol means 31A controls the operation of the proportional valve 36for forward travel, or proportional valve 37 for reverse travel suchthat the pump swash plate 16A is operated at the target operating speedset by the first operating speed setting means 31B. By this controloperation, hunching can be controlled while improving response propertyof the pump swash plate 16A to the operation of speed change pedal 24.As a result, the vehicle speed can reach the speed set by the operatedposition of speed change pedal 24 quickly and accurately. Since theoperating speed of pump swash plate 16A in reverse travel is less thanthe operating speed of pump swash plate 16A during a forward travel, andthe speed change operation of the hydrostatic type continuously variablespeed change device 10 in a reverse travel is performed more graduallycompared with a speed change operation of the hydrostatic typecontinuously variable speed change device 10 during a forward travel, itbecomes easy to perform speed change operation of hydrostatic typecontinuously variable speed change device 10 during a reverse travelwhen it is difficult to have a sense of speed compared with during aforward travel.

The control device 31 has data change means 31C provided with thecontrol program which changes the map data which the first operatingspeed setting means 31B uses. As described below, data change means 31Cis set up such that the map data which the first operating speed settingmeans 31B uses is changed appropriately according to various situations.

The data change means 31C changes the map data which correlates thedeviation of pump swash plate 16A with the operating speed of pump swashplate 16A based on the operated position of regulation dial (an exampleof an operator-manipulated control) 42 which has a potentiometer in theoperator's station 8 such that as the regulation dial 42 is operatedmore toward the quick side from the reference position, operation ofpump swash plate 16A is performed more promptly by changing the map suchthat the deviation of pump swash plate 16A corresponds with theoperating speed of pump swash plate 16A such that the operating speed ofpump swash plate 16A for a given deviation of the pump swash plate 16Abecomes more rapid. Also, as the regulation dial 42 is operated moretoward a slow side from the reference position, the operating speed ofpump swash plate 16A to the deviation of pump swash plate 16A is less sothat the operation of pump swash plate 16A is performed more gradually.

That is, by operating the regulation dial 42, a response to a gearchange operation of the hydrostatic type continuously variable speedchange device 10 by speed change pedal 24 may be changed according tothe liking of a driver, resulting in an improved speed changecharacteristics.

The data change means 31C changes the map data based on detectedinformation from oil temperature sensor 43 by which the temperature ofthe hydraulic fluid supplied to regulator valve 28 is detected, suchthat operating speed of pump swash plate 16A is correlated with a givendeviation of pump swash plate 16A so that the operating speed of pumpswash plate 16A for the deviation of pump swash plate 16A is gradual inresponse to a fall of oil temperature so that the lower the temperatureof hydraulic fluid is, more gradually the operation of pump swash plate16A is performed.

That is, the system takes into consideration that the viscosity ofhydraulic fluid becomes high and the response of pump swash plate 16Abecomes more sluggish with a fall of the hydraulic fluid temperature.The target operating speed of pump swash plate 16A is set lower with alower temperature of hydraulic fluid. This can help prevent hunchingresulting from the fall in the response of the pump swash plate 16A,which is more likely to happen when temperature of hydraulic fluid isnot taken into consideration and when the temperature of hydraulic fluidis low.

The data change means 31C changes the map data which correlates thedeviation of pump swash plate 16A with the operating speed of pump swashplate 16A based on detected information from sub-gear change sensor (anexample of a gear ratio detection means) 45 which has a potentiometerwhich detects the gear ratio of the gear type speed change device 11based on the operation position of sub-gearshift lever 44 in theoperator's station 8, such that the higher the gear ratio of the geartype speed change device 11 is, the greater is the speed at which theoperation of pump swash plate 16A is performed. Therefore, the means 31Cmodifies the map that correlates the deviation of pump swash plate 16Awith the operating speed of pump swash plate 16A such that the operatingspeed of pump swash plate 16A for a given deviation of pump swash plate16A is greater in response to increase in the gear ratio of the geartype speed change device 11.

That is, the target operating speed of pump swash plate 16A is set at ahigher speed in consideration of the fact that the reaction to operationof pump swash plate 16A becomes slower, as the gear ratio of gear typespeed change device 11 is set to the higher speed side. Thus,irrespective of the gear ratio of gear type speed change device 11, theresponse during a gear change operation of the hydrostatic typecontinuously variable speed change device 10 by speed change pedal 24 isconsistent.

The data change means 31C changes the map to be used to data whichcorrelates the deviation of pump swash plate 16A with the operatingspeed of pump swash plate 16A such that the slower the operating speedof speed change pedal 24 is, the more gradually the pump swash plate 16Ais operated, based on the detected information from the operating speeddetection means 46 which detects the operating speed of speed changepedal 24, such that with a decrease in the operating speed of speedchange pedal 24, the operating speed of the pump swash plate 16A for agiven deviation of pump swash plate 16A becomes more gradual. Theoperating speed of speed change pedal 24 is obtained by differentiatingthe output of pedal sensor 29 with respect to time. Therefore, thedetection means 46 is considered to have the pedal sensor 29 and thecontrol device 31.

As a result, even if the speed change pedal 24 is operated very slowly,the movement of the pump swash plate 16A lags behind the operation ofspeed change pedal 24. Thus, since the possibility that operation ofpump swash plate 16A follows the operation of speed change pedal 24causing a step-wise speed change can be avoided, a smooth gear changeoperation of hydrostatic type continuously variable speed change device10 by speed change pedal 24 can be performed irrespective of theoperating speed of speed change pedal 24.

The operating speed detection means 46 has the pedal sensor 29 and theoperation program, which data change means 31C has, to calculate theoperating speed of speed change pedal 24 based on detected informationfrom the pedal sensor 29.

The data change means 31C changes the map data to be used into the mapdata which correlates the deviation of pump swash plate 16A with theoperating speed of pump swash plate 16A based on detected informationfrom swash plate sensor 30 such that when it is detected that theoperation position of pump swash plate 16A is near the neutral positionor at the neutral position, operation of pump swash plate 16A isperformed gently so that the operating speed of pump swash plate 16A fora given deviation of pump swash plate 16A is restricted to the low speedside.

As a result, when pump swash plate 16A is located near the neutralposition or at the neutral position, even if the speed change pedal 24is suddenly depressed, since pump swash plate 16A is not quicklyoperated in the speed increase direction with the step in operation, thepump swash plate 16A is operated in the speed increase direction gently.Thus even if the step in operation of the speed change pedal 24 iscarried out very rapidly, a smooth start is ensured without a suddenstart or a sudden acceleration from a very slow speed.

The data change means 31C changes the used map data which correlates thedeviation of pump swash plate 16A with the operating speed of pump swashplate 16A based on the detected information from rotation sensor (anexample of a load detection means) 47 which detects the enginerotational speed, and the target operation position of pump swash plate16A set up by pump swash plate control means 31A, such that when thetarget operation position of pump swash plate 16A is set to the lowspeed side when the engine rotational speed is low, operation of pumpswash plate 16A is performed quickly, such that, in response to the fallof the engine rotational speed, the operating speed of pump swash plate16A for a given deviation of pump swash plate 16A becomes greater. Themap data is changed such that when it is detected that the targetoperation position of pump swash plate 16A was set to the high speedside when the engine rotational speed is raising, the operation of pumpswash plate 16A is performed gently so that the operating speed of pumpswash plate 16A for a given deviation of the pump swash plate 16Abecomes slow in response to the rise of the engine rotational speed.

If the speed change pedal 24 is operated to the decelerating side whenthe engine rotational speed is low due to increase of traveling loadetc., the pump swash plate 16A is operated quickly to the deceleratingdirection in response to the operation, mitigating the excess engineload. This reduces the problem of an engine stall in spite of the speedchange pedal 24 operated in the decelerating direction because reductionof the engine load is too slow due to slow response of the hydrostatictype continuously variable speed change device 10. When the speed changepedal 24 is operated to the accelerating side when the engine rotationalspeed is raising due to reduction of traveling load etc., sinceoperation to the speed-increase direction of pump swash plate 16A basedon the operation is performed gently, the rapid increase of the vehiclespeed, resulting from the rapid speed increase operation of the pumpswash plate 16A with the increase in the engine rotational speed, isavoided. That is, gear change operation of hydrostatic type continuouslyvariable speed change device 10 by speed change pedal 24 can beperformed well irrespective of change in the engine rotational speed.

The data change means 31C changes the map data which correlates thedeviation of pump swash plate 16A with the operating speed of pump swashplate 16A based on detected information from the brake sensor (anexample of a braking detecting means) 49 which has a potentiometer whichdetects the operation of brake mechanism (not shown) from the operationposition of brake pedal 48 in the operator's station 8, such that whenbrake mechanism is carrying out the braking operation, operation to thedecelerating direction of pump swash plate 16A is performed promptlysuch that the operating speed of pump swash plate 16A for a givendeviation of pump swash plate 16A becomes greater, taking intoconsideration the fall of the vehicle speed by the braking operation ofbrake mechanism.

This prevents the interference between the hydrostatic type continuouslyvariable speed change device 10 and the brake mechanism during thebraking operation when the step in operation of speed change pedal 24 isceased and the step in operation of the brake pedal 48 is performed.Thus this helps increase operating life of the hydrostatic typecontinuously variable speed change device 10 as well as the brakemechanism.

The data change means 31C changes the map data which correlates thedeviation of pump swash plate 16A with the operating speed of pump swashplate 16A based on the detected information from the vehicle speedsensor (speed detecting means) 50 which detects the vehicle speed fromthe output rotational speed of gear type speed change device 11, suchthat when the vehicle speed is low, the operating speed of pump swashplate 16A for a given deviation of pump swash plate 16A is performedslowly so that operation of pump swash plate 16A is gradual in responseto the fall of the vehicle speed.

When traveling at low speed, the response of pump swash plate 16A to anoperation of speed change pedal 24 becomes less sensitive, making iteasier to perform an inching operation of the vehicle speed required atlow speed.

The data change means 31C changes the map data which correlates thedeviation of the pump swash plate 16A with the operating speed of thepump swash plate 16A based on detected information from the swash platesensor 30, and the target operation position of pump swash plate 16A setby the pump swash plate control means 31A, such that when the targetoperation position of the pump swash plate 16A is set to the speedincrease side relative to the actual operation position, operation ofpump swash plate 16A is performed more gently, and such that when thetarget operation position of pump swash plate 16A is set to the slowdownside relative to the actual operation position, operation of pump swashplate 16A is performed promptly, and such that when the neutral positionis between the target operation position of pump swash plate 16A and theactual operation position, operation of pump swash plate 16A isperformed much more promptly.

Therefore, depending on the relationship of the target operationposition of pump swash plate 16A, and the actual operation position, theoperating speed of pump swash plate 16A for a given deviation of pumpswash plate 16A changes.

The responses to the speed increase operation and slowdown operation ofthe hydrostatic type continuously variable speed change device 10 byspeed change pedal 24 may be made different by this method. Since theincrease of the engine load by a speed increase operation of hydrostatictype continuously variable speed change device 10 is reduced and theengine load by a slowdown operation of hydrostatic type continuouslyvariable speed change device 10 is reduced, an engine stall due to agear change operation of the hydrostatic type continuously variablespeed change device 10 by the speed change pedal 24 during high load canbe effectively prevented. Since the operation delay of the pump swashplate 16A in response to an operation of the speed change pedal 24 isprevented, speed change operation of the hydrostatic type continuouslyvariable speed change device 10 in the forward or the reverse directionby the speed change pedal 24 can be performed comfortably.

The data change means 31C changes the map data which correlates thedeviation of the pump swash plate 16A with the operating speed of pumpswash plate 16A based on detected information from swash plate sensor30, and the target operation position of the pump swash plate 16A set bythe pump swash plate control means 31A such that at the start of thevehicle movement where the pump swash plate 16A is operated in the speedincrease direction from the neutral position, the operation of pumpswash plate 16A is performed gently, such that the operating speed ofpump swash plate 16A for a given deviation of pump swash plate 16Abecomes slower at the start of travel and such that when the vehicle isstopped where a slowdown operation of the pump swash plate 16A iscarried out from a speed increase position to the neutral position,operation of the pump swash plate 16A is performed with greater speed,so that at the time of the vehicle stop, the operating speed of pumpswash plate 16A for a given deviation of pump swash plate 16A becomesquicker.

This allows the changes in the response to the traveling start and atraveling stop by operation of speed change pedal 24. This also helpsprevent a possible sudden acceleration at the start of the vehiclemovement.

As the map data of the first operating speed setting means 31B, a mapdata for the forward travel for the gear change operating speed at thetime of a forward travel, and a map data for the reverse travel for thegear change operating speed at the time of a reverse travel may beseparately provided so that the data change means 31C has different mapdata to be used based on detected information from the pedal sensor(forward reverse travel detection means) 29.

When the forward travel decelerating spring 32 and the reverse traveldecelerating spring 33 perform neutral return operation (slowdownoperation) of the pump swash plate 16A, the inertia at the time of atrailer operation etc. may make it hard to perform the slowdownoperation toward the neutral position of pump swash plate 16A despitethe slowdown operation of speed change pedal 24.

Thus despite the deceleration operation of the speed change pedal 24based on detected information from the pedal sensor 29, and detectedinformation from the swash-plate sensor 30, when it is detected thatdeceleration operation of the pump swash plate 16A with the forwardtravel decelerating spring 32 or the reverse travel decelerating spring33 is not performed, the pump swash plate control means 31A controls theactuation of the proportional valve 36 for forward travel or theproportional valve 37 for reverse travel on the side opposite to theside used to actuate the pump swash plate 16A to the present actuationposition. Thus the pump cylinder 26 is forced to operate in thedirection in which deceleration operation of the pump swash plate 16A iscarried out toward the neutral position.

As a result, even if the pump swash plate 16A does not deceleratedespite the rate at which the speed change pedal 24 is operated isslowed down due to inertia for example during a trailer operation etc.,deceleration operation of the pump swash plate 16A can be carried out bya forced operation of the pump cylinder 26 by the operation control ofthe proportional valve 36 for forward travel or the proportional valve37 for reverse travel by the pump swash plate control means 31A.

The engine stall performance of the work vehicle carrying thehydrostatic type continuously variable speed change device 10 is shownin FIG. 9(A). This engine stall performance is determined by the outputtorque of an engine 1, the pressure of the hydrostatic type continuouslyvariable speed change device 10, and the actuation position (swash-plateangle) of the pump swash plate 16A.

The line L shown in FIG. 9(A) is an engine stall performance line wherethe engine rotational speed and the shift operation position of the pumpswash plate 16A balance at the time of the maximum load to thehydrostatic type continuously variable speed change device 10 at which ahigh pressure relief of the hydrostatic type continuously variable speedchange device 10 releases pressure due to running load.

In FIG. 9(A), the engine rotational speed is shown with respect to theset rotating speed at 100%, and the actuation position of the pump swashplate 16A shown with respect to 100% for the maximum acceleratingposition (the maximum swash-plate angle).

The engine stall performance of the work vehicle carrying thehydrostatic type continuously variable speed change device 10 isexplained with reference to FIG. 9(A). When the operation load fordriving implements etc. other than running load is applied to the engine1 with the pump swash plate 16A held at a certain actuation position, aspoint a shows, the engine rotational speed and the actuation position ofthe pump swash plate 16A may go into the region Za inside the enginestall performance line L.

In this case, when the operation load other than running load etc. isstable, even if running load increases, the engine rotational speed doesnot fall. But if the operation load other than running load etc.increases, the engine rotational speed will drop and an engine 1 willstop or stall.

With the pump swash plate 16A held at a certain actuation or operationposition, as shown at point b, when the engine rotational speed and theactuation position of the pump swash plate 16A are located in the regionZb outside the engine stall performance line L, if running loadincreases, the engine rotational speed will fall and an engine 1 willstop.

With the pump swash plate 16A held at a certain actuation position, whenthe engine rotational speed and the actuation position of the pump swashplate 16A are located in region Zc, as shown by point c, if the runningload increases, the engine rotational speed falls until it arrives atthe engine stall performance line L. As it reaches the engine stallperformance line L, the engine rotational speed will be stabilized.

The engine stall performance line L is such that as the output of theengine 1 decreases, the actuation position of the pump swash plate 16Afor a given engine rotational speed in FIG. 9(A) becomes smaller.

That is, the smaller the output of the engine 1 is, more likely theengine stall becomes due to overload when accelerating by stepping inthe speed change pedal 24 when traveling with large running load, orwhen climbing up a hill.

To solve this problem, as shown in FIG. 6, the control device 31 has theautomatic pump swash plate control means 31D which changes the actuationposition of the pump swash plate 16A based on the engine load.

As shown in FIGS. 6 and 9(B), the automatic pump swash plate controlmeans 31D has an operation program which computes the decrease amount(engine drop amount) from the set rotating speed of the enginerotational speed based on detected information from the setting rotationsensor (an example of a set-rotating-speed detection means) 52 which hasa potentiometer which detects the set rotating speed of an engine 1 fromthe actuation position of the accelerator lever 51 in the operator'sstation 8, and on detected information from the rotation sensor 47, aplurality of map data (an example of correlation data) which correlatesthe engine rotational speed with the actuation position of the pumpswash plate 16A, and, a control program which operates the pump swashplate 16A by controlling actuation of the proportional valve 36 forforward travel, or the proportional valve 37 for reverse travel based onthe calculation result and the map data of the operation program.

Each map data of the automatic pump swash plate control means 31D isdetermined based on the engine stall performance line L. And the enginerotational speed and the actuation position of the pump swash plate 16Aare correlated such that when the engine rotational speed falls to thepredetermined engine rotation region h, the lower the rotation rate is,the closer the limit operation position is to the neutral position andsuch that the limit operation position of the pump swash plate 16A isnot set at the neutral position (see FIG. 9(B)).

To explain in more detail, as shown in FIG. 9(B), in the first region h1where the engine drop amount is small among the predetermined enginerotation regions, the engine rotational speed and the actuation positionof the pump swash plate 16A are correlated such that the amount ofchange of the pump swash plate 16A is large for a given amount of changeof the engine rotational speed.

In the second region h2 where an engine drop amount is larger than thefirst region h1, the engine rotational speed and the actuation positionof the pump swash plate 16A are correlated such that the amount ofchange of the pump swash plate 16A is small for a given amount of changeof the engine rotational speed and so as to have a stabilizing point pwhere the engine rotational speed does not fall due to running load.

The engine rotational speed and the actuation position of the pump swashplate 16A are correlated in the third region h3 where an engine dropamount is larger than the second region h2, such that the amount ofchange of the pump swash plate 16A is large for the given amount ofchange of the engine rotational speed.

The control program of the automatic pump swash plate control means 31Dsets the actuation position of the pump swash plate 16A corresponding tothe engine drop amount which the operation program computed as the limitoperation position of the pump swash plate 16A based on the calculationresult and map data of an operation program, and controls actuation ofthe proportional valve 36 for forward travel or the proportional valve37 for reverse travel based on the set limit operation position anddetected information from the swash-plate sensor 30, such that the limitoperation position of the pump swash plate 16A comes to agreement withthe actual actuation position.

That is, with the automatic pump swash plate control means 31D, when theengine rotational speed falls to the first region h1 due to an increasein the engine load, the load control is performed where driving torqueis increased while preventing an engine stall by returning the pumpswash plate 16A to the slowdown direction greatly, and reducing theengine rotational drop speed.

When the engine rotational speed falls to the second region h2 despitethis load control, the load control can be implemented that prevents anengine stall while prioritizing increasing of drive torque by loweringthe decelerating operation amount of the swash plate 16A and increasingthe drive torque while allowing the operator to feel the engine load. Ifthe load on the engine is running load, the engine rotation rate willstop decreasing below the stabilizing point p once the rate reaches thepoint p.

The difference between FIG. 9B and FIG. 9C is that the vertical axis inFIG. 9C represents engine rotation rate. Max indicates the setting ofthe map data for large load and IDL indicates the setting for map datafor light load.

When the engine rotational speed falls to the third region h3 due toload other than running load, for example, from a lift actuation of aliftable implement, load control can be implemented that secures drivingtorque while giving priority to prevention of the engine stall, byreturning the pump swash plate 16A in the slowdown direction by a largeamount, and reducing the lowering speed of the engine rotational speed.

With this control, a load control that prevents an engine stall due tooverload can be performed during a loader operation where a front loaderA is connected to the tractor or during a tilling operation where thetilling apparatus is connected to the tractor etc., even if the operatorperforms shift operation without consideration to an operation load etc.Thus, improvement in the response in the system may be expected.

Moreover, since the limit operation position of the pump swash plate 16Ais never set at the neutral position, the pump swash plate 16A is notreturned to the neutral position by this load control. Therefore, apossibility of the pump swash plate 16A returning to the neutralposition, and the vehicle unintentionally starting to run backwards bythe load control during an uphill climb will be avoided.

The control device 31 has a plurality of map data that correlates therate of change of the engine rotational speed with the operation speedof the pump swash plate 16A. It has a second operation speed settingmeans 31E that has a control program which sets the target operationspeed for the pump swash plate 16A, based on those map data and detectedinformation from a variation speed detection means 53 that detects therate of change of the engine rotational speed.

Each map data of the second operation speed setting means 31E correlatesthe rate of change of the engine rotational speed with the operationspeed of the pump swash plate 16A such that the greater the rate ofchange of the engine rotational speed is, greater the operation speed ofthe pump swash plate 16A becomes.

The control program of the second operation speed setting means 31E setsthe operation speed of the pump swash plate 16A corresponding to therate of change of the engine rotational speed which the rate of changedetection means 53 detected as the target operation speed of the pumpswash plate 16A, based on the memorized map data and detectedinformation from the variation speed detection means 53 and the settarget operation speed is outputted to the automatic pump swash platecontrol means 31D.

The control program of the automatic pump swash plate control means 31Dcontrols actuation of the proportional valve 36 for forward travel, orthe proportional valve 37 for reverse travel such that the pump swashplate 16A is operated at the target operation speed set by the secondoperation speed setting means 31E. Thus, a good load control taking intoconsideration the rate of change of the engine rotational speed ispossible. Despite changes in the rate of change of the engine rotationalspeed, driving comfort is maintained during a drop or increase in enginerotation speed. Moreover, deceleration operation of the pump swash plate16A to lowering of the engine rotational speed can be performed withsufficient response, and the engine stall resulting from the actuationdelay of the pump swash plate 16A can be prevented.

The rate of change detection means 53 has the rotation sensor 47, andthe operation program of the second operation speed setting means 31Ethat computes the rate of change of the engine rotational speed based ondetected information from the rotation sensor 47.

The data change means 31C has a control program which changes the mapdata which is used by the automatic pump swash plate control means 31Dand the second operation speed setting means 31E. The data change means31C changes the map data which the automatic pump swash plate controlmeans 31D uses to a map data that correlates the engine rotational speedwith the actuation position of the pump swash plate 16A, based ondetected information from the set rotation sensor 52, such that thesmaller the set rotating speed of an engine 1, the greater the controlamount of the pump swash plate 16A for a given change in the enginerotational speed (see FIG. 9). It also changes the map data, which thesecond operation speed setting means 31E uses, to a map data thatcorrelates the rate of change of the engine rotational speed with theoperation speed of the pump swash plate 16A, based on detectedinformation from the sub-gear change sensor 45, such that the lower thegear ratio of the gear type speed change device 11 is, the quicker theactuation of the pump swash plate 16A becomes, and such that theoperation speed of the pump swash plate 16A becomes greater for a rateof change of the engine rotational speed with lowering of the gear ratioof the gear type speed change device 11.

That is, a suitable load control is chosen based on the map data inconsideration of the set rotating speed of the engine 1. Therefore,irrespective of the set rotating speed of an engine 1, the engine stalldue to overload can be effectively prevented.

Depending on the gear ratio of the gear type speed change device 11which is shifted to a low speed side when performing an operation with alarger load, the map data is set such that the lower the gear ratio is,the greater the operation speed of the pump swash plate 16A for a givenrate of change of the engine rotational speed. Therefore, decelerationoperation of the pump swash plate 16A can be promptly performed inresponse to a rapid reduction in engine rotational speed during anoperation with a large load. As a result, the engine stall due tooverload can be reliably prevented.

While not shown, a manually operated operation speed setting device(operation speed setting means) is provided and has a potentiometer or aswitch for setting the target operation speed of the pump swash plate16A during a load control etc. The automatic pump swash plate controlmeans 31D may be set up such that it controls actuation of theproportional valve 36 for forward travel, or the proportional valve 37for reverse travel such that the pump swash plate 16A is operated at thetarget operation speed set by this operation speed setting device inconsideration of the operation load which changes depending on the kindof the attached implement.

An operating tool for a data change command (commanding means) which hasa potentiometer, a switch, etc., may be provided to the data changemeans 31C in the operator's station 8, for commanding a change of themap data which the automatic pump swash plate control means 31D or thesecond operation speed setting means 31E uses. This allows the map datato be changed depending on the attached implement.

This tractor is equipped with a switching mechanism 54 which switchesthe swash plate (a motor swash plate) 17A of the variable capacity motor17 between high and low positions.

The switching mechanism 54 has a hydraulic cylinder 55 which operatesthe motor swash plate 17A, a changeover valve 56 which controls flow ofthe hydraulic fluid to this cylinder 55, an electromagnetic controlvalve 57 which operates this changeover valve 56, a high pressureselection valve 58 which enables feeding of the hydraulic fluid from theclosed circuit 20 of the hydrostatic type continuously variable speedchange device 10 to this control valve 57, a switching lever 59 arrangedat the lower left position with respect to the steering wheel 6, a leversensor 60 which has a switch which detects the actuation position ofthis switching lever 59, and motor swash-plate control means 31F whichthe control device 31 has as a control program which performshigh-to-low switch actuation of the motor swash plate 17A based ondetected information from this lever sensor 60.

The cylinder 55 for motor and the variable capacity motor 17 areremovably housed within the second casing part 4B of the transmissioncase 4.

When the switching lever 59 is operated to a low-speed position based ondetected information from a lever sensor 60, the motor swash-platecontrol means 31F performs high-to-low switching control which switchesthe motor swash plate 17A from the high-speed position to the low-speedposition, and turns on the corresponding indicating lamp 61. And, whenthe switching lever 59 is operated in a high-speed position, it performslow-to-high switching control which switches the motor swash plate 17Afrom the low-speed position to the high-speed position, and turns on thecorresponding indicating lamp 62.

That is, when the vehicle speed drops substantially due to increase inload during an up hill climb or a field operation with the switchinglever 59 set in the high-speed position, the driving force to the rightand left front wheels 3 and the right and left rear wheels 5 can beincreased by switching the switching lever 59 to a low-speed positionfrom a high-speed position so that the vehicle can keep on climbing orcontinue with work operation.

The indicating lamps 61 and 62 are arranged in the console panel 63arranged under the steering wheel 6.

The motor swash-plate control means 31F stores the present actuationposition of the pump swash plate 16A in the high-to-low switchingcontrol based on detected information from the swash-plate sensor 30,and calculates a slowdown target actuation position of the pump swashplate 16A, and controls actuation of the proportional valve 36 forforward travel or the proportional valve 37 for reverse travel so thatdeceleration operation of the pump swash plate 16A is carried out at apredetermined operation speed to the computed slowdown target actuationposition. The means 31F controls the actuation of the proportional valve36 or 37 and the control valve 57 such that after the pump swash plate16A arrives at the slowdown target actuation position, an acceleratedreturn of the pump swash plate 16A to the stored actuation position atthe predetermined operation speed and the switch over operation of themotor swash plate 17A from the high speed position to the low speedposition at a predetermined speed are perfumed simultaneously (see FIG.10(A)).

Moreover, in the low-to-high switching control, based on detectedinformation from the swash-plate sensor 30, the present actuationposition of the pump swash plate 16A is stored, and the slowdown targetactuation position of the pump swash plate 16A is computed. Actuation ofthe proportional valve 36 for forward travel or the proportional valve37 for reverse travel and actuation of the control valve 57 arecontrolled such that deceleration operation of pump swash plate 16A tothe computed slowdown target actuation position at the operation speedand switch actuation from a low-speed position to the high-speedposition of the motor swash plate 17A at the operation speed areperformed simultaneously. Afterward, actuation of the proportional valve36 for forward travel or the proportional valve 37 for reverse travel iscontrolled so that the pump swash plate 16A is returned to the memorizedactuation position at the predetermined speed. [See FIG. 10(B)]

That is, when switching the motor swash plate 17A to a low-speedposition from a high-speed position, by performing not only theswitching actuation but also accelerating actuation of the pump swashplate 16A simultaneously, the capacity variation in the variablecapacity motor 17 generated by the switching actuation to the low-speedposition of the motor swash plate 17A from a high-speed position can beoffset by the capacity variation in the variable capacity pump 16generated by accelerating actuation of the pump swash plate 16A.Moreover, when switching the motor swash plate 17A to a high-speedposition from a low-speed position, by performing not only the switchactuation but also a deceleration operation of the pump swash plate 16Asimultaneously, the capacity variation in the variable capacity motor 17generated by the switching actuation to the high-speed position of themotor swash plate 17A from a low-speed position can be offset by thecapacity variation in the variable capacity pump 16 generated inconnection with the deceleration operation of the pump swash plate 16A.Therefore, the shift shock generated by a switching actuation of themotor swash plate 17A can be alleviated.

In addition, while not shown, a high speed response valve may be usedinstead of the changeover valve 56. When the motor swash-plate controlmeans 31F performs switch actuation of the motor swash plate 17A, thehigh speed response valve is duty-controlled such that the operationspeed of the motor swash plate 17A may fall so as to slow any capacityvariation in the variable capacity motor 17 generated when switching themotor swash plate 17A, which alleviates the shift shock resulting fromthe capacity variation in the variable capacity motor 17.

When traveling with the motor swash plate 17A switched to the high-speedposition, the motor swash-plate control means 31F may perform thehigh-to-low switching control in conjunction with braking actuation of abraking system based on detected information from the brake sensor 49 soas to improve braking operation.

Also, the motor swash-plate control means 31F may be configured suchthat it performs the high-to-low switching control when traveling withthe motor swash plate 17A switched to the high-speed position based ondetected information from the pedal sensor 29, and detected informationfrom the swash-plate sensor 30, irrespective of the decelerationoperation of the speed change pedal 24, when it is detected thatdeceleration operation of the pump swash plate 16A with the forwardtravel decelerating spring 32 or the reverse travel decelerating spring33 is not performed so that the high-to-low switching control canperform a deceleration operation if deceleration operation of the pumpswash plate 16A is not carried out due to inertia during a traileroperation etc., despite the decelerating operation of the speed changepedal 24.

Furthermore, the motor swash-plate control means 31F may be configuredso that it performs the high-to-low switching control based on detectedinformation from the operation speed detection means 46, when the motorswash plate 17A is switched to the high-speed position, and when theoperation speed of the speed change pedal 24 is greater than apredetermined operation speed so as to prevent an unexpected start andsudden acceleration.

As shown in FIG. 1, the switching lever 59 is installed such that theoperable end is located near the left part of the steering wheel 6. Thisallows a high-to-low switch operation of the motor swash plate 17Awithout lifting a hand off the steering wheel 6. Moreover, when thefront loader A (refer to FIG. 6) is connected to the tractor, thehigh-to-low switch actuation of the motor swash plate 17A can beperformed without lifting the hand from the control lever for frontloader actuation (not shown) arranged on the right-hand side of asteering wheel 6.

As shown in FIGS. 4-6, the control device 31 has an automatic motorswash-plate control means 31G as a control program. Automatic motorswash-plate control means 31G performs automatic high-to-low switchingcontrol which switches the motor swash plate 17A to the low-speedposition from the high-speed position, and turns on the correspondingindicating lamp 61 when it is detected based on detected informationfrom the pedal sensor 29 that the actuation position of the speed changepedal 24 is operated to the predetermined actuation position or theoperation area, when the engine rotational speed is detected to havefallen to the low-to-high switch engine speed near a predeterminedmaximum torque output rotational frequency, or the low-to-high switchengine-speed region set for a given actuation position of the speedchange pedal 24 based on the maximum torque output characteristic of anengine 1, and the engine drop amount computed by the operation programof the automatic pump swash plate control means 31D. The automatic motorswash-plate control means 31G also performs the automatic low-to-highswitching control which switches the motor swash plate 17A to ahigh-speed position from a low-speed position, and turns on thecorresponding indicating lamp 62, when it is detected based on an enginedrop amount that the engine rotational speed went up to the low-to-highswitch engine speed near a predetermined set rotating speed or thelow-to-high switch engine-speed region, and when actuation of the speedchange pedal 24 to the actuation position or operation area set upbeforehand is detected based on detected information from the pedalsensor 29.

More specifically, if the actuation position of the pedal 24, whosemaximum treading-in position is 100%, is 50% or less, for example,automatic high-to-low switching control is performed when the enginerotational speed falls to 85% of the engine speed. If the actuationposition of the pedal 24 is 90% or greater, the automatic high-to-lowswitching control is performed when the engine rotational speed falls to70% or less. When the engine rotational speed goes up to 90% or greater,automatic low-to-high switching control is performed when the actuationposition of a pedal 24 is operated to be 80% or greater.

That is, a large load condition is naturally assumed when the enginedrop amount is large when the speed change pedal 24 is operated by agreater amount. But also if a certain amount of engine drop occurs whenthe amount of the step-in operation of the speed change pedal 24 issmall, a large load condition is assumed where a greater drop in enginerotation is expected with a large step-in operation of the pedal 24.Therefore, even if an engine drop amount is small, the high-to-lowswitching control is performed to secure sufficient driving force.Therefore, even if the operator does not perform shift operation takinginto consideration an operation load etc., heavy-load operation whichrequires a large driving force may be continued without an engine stall.Just when the operator tends to want to accelerate further as the loadfalls and the engine rotational speed goes up to near predetermined setrotating speed, low-to-high switching control is performed and thevehicle speed is raised. This avoids the inconvenience that low-to-highswitching control is performed resulting in an unexpected accelerationdespite the operator decreasing the amount of step-in operation on thespeed change pedal 24 to slow down with a decrease in the load.

In the high-to-low switching control of the automatic motor swash-platecontrol means 31G, when the switch actuation is performed, the vehiclespeed is low due to traveling load and a shift shock is assumed to besmall. Therefore, control for alleviating a shift shock as in thehigh-to-low switching control in the motor swash-plate control means 31Fis not performed. Actuation of the proportional valve 36 for forwardtravel or the proportional valve 37 for reverse travel is controlled sothat switch actuation of the motor swash plate 17A is carried out from ahigh-speed position to a low-speed position at the predeterminedoperation speed. After switching the motor swash plate 17A to alow-speed position, that state is maintained for a predetermined timeperiod (for example, for 2 seconds).

Also, in the low-to-high switching control of the automatic motorswash-plate control means 31G, the same control actuation as in thelow-to-high switching control in the motor swash-plate control means 31Fis performed, and the shift shock generated by low-to-high switchactuation of the motor swash plate 17A is alleviated. After switchingthe motor swash plate 17A to a high-speed position, this state ismaintained for a predetermined time period (for example, for 2 seconds).

It is also possible to configure the automatic motor swash-plate controlmeans 31G such that it performs the automatic high-to-low switchingcontrol which switches the motor swash plate 17A from the high-speedposition to the low-speed position and turns on the correspondingindicating lamp 61, as the engine rotational speed is detected to havefallen to the low-to-high switch engine speed near a predeterminedmaximum torque output rotational speed, or to the low-to-high switchengine-speed region, based on the maximum torque output characteristicsof the engine 1, and the engine drop amount computed by the operationprogram of the automatic pump swash plate control means 31D and suchthat it performs an automatic low-to-high switching control whichswitches the motor swash plate 17A from the low-speed position to ahigh-speed position, as the engine rotational speed is detected to havegone up to the low-to-high switch engine speed near predetermined setrotating speed, or to the low-to-high switch engine-speed region, basedon an engine drop amount, and turns on the corresponding indicating lamp62.

The control device 31 has mode change-over means 31H as a controlprogram which switches the control mode performed based on actuation ofthe mode setting device 64 which has a normally open switch on thedisplay panel 63. When an ON signal is inputted in connection with thepressing of the mode setting device 64, the mode change-over means 31Hswitches the transmission control mode between a manual-control mode, asemi-automatic-control mode, or a automatic-control mode and turns on anindicating lamps 65-67 corresponding to each control mode. In themanual-control mode, it performs the speed control using controlactuation of the pump swash plate control means 31A and switchingcontrol using control actuation of the motor swash-plate control means31F. In the semi-automatic-control mode, it performs the speed controlwhich uses control actuation of the pump swash plate control means 31A,and a load control using control actuation of the automatic pump swashplate control means 31D, and a switching control using control actuationof the motor swash-plate control means 31F, such that priority may begiven to load control as opposed to the speed control. In theautomatic-control mode, the mode change-over means 31H performs a speedcontrol which uses control actuation of the pump swash plate controlmeans 31A, and a load control using control actuation of the automaticpump swash plate control means 31D, and the automatic switching controlusing control actuation of the automatic motor swash-plate control means31G, such that priority is given to load control as opposed to speedcontrol, and such that load control and automatic switching control arecoordinated appropriately.

That is, when manual-control mode is selected, the pump swash plate 16Ais operated based on the actuation position of the speed change pedal 24etc., so as to arrive at the target actuation position corresponding tothe actuation position of the speed change pedal 24 with targetoperation speed. And the motor swash plate 17A will be switched betweenthe high and low positions based on actuation of the switching lever 59.

The pump swash plate 16A is operated based on the actuation position ofthe speed change pedal 24 etc., so as to arrive at the target actuationposition corresponding to the actuation position of the speed changepedal 24 with target operation speed. And when a drop in engine speedoccurs, based on an engine drop amount etc., it is operated so as toarrive at the limit operation position corresponding to an engine dropamount at the target operation speed, and the motor swash plate 17A isswitched between the high and low positions based on actuation of theswitching lever 59.

When automatic-control mode is selected, the pump swash plate 16A isoperated so as to arrive at the target actuation position correspondingto the actuation position of the speed change pedal 24 at the targetoperation speed. When engine speed drops, the pump swash plate 16A isoperated so as to arrive at the limit operation position correspondingto an engine drop amount at target operation speed, based on an enginedrop amount etc. And the motor swash plate 17A is switched between thehigh and low positions at a suitable timing based on the actuationposition of the speed change pedal 24, or the engine drop amount, etc.

Therefore, if the manual-control mode is selected for example, fortraveling and a light load operation, or if the semi-automatic-controlmode is selected when climbing a relatively steep hill, and for amedium-load operation, or if the automatic-control mode is selected whenclimbing a very steep hill, or for a heavy-loading operation, suchtraveling and operation can be performed without increased burden to theoperator.

Incidentally, in the load control in the automatic-control mode, thelower limit engine speed of an engine 1 is set lower than the loadcontrol in the semi-automatic-control mode so that control sensibilityis set low which tends to cause drops in engine speeds. Therefore, theautomatic switching control which switches the motor swash plate 17A toa low-speed position can be easily performed.

Moreover, when switch actuation to the low-speed position of the motorswash plate 17A based on actuation of the switching lever 59 isperformed in the automatic-control mode, since it is impossible toperform switch actuation to the low-speed position of the motor swashplate 17A by automatic switching control, control mode switches fromautomatic-control mode to semi-automatic-control mode automatically.

As shown in FIG. 6, the console panel 63 has the liquid-crystal-displaydevice 69 where the display may be changed among a vehicle speeddisplaying mode, or a remaining fuel displaying mode, etc. based onactuation of the display change-over switch 68. Thisliquid-crystal-display device 69 displays the target actuation position69A or the limit operation position 69B, and the current position 69C ofthe pump swash plate 16A which changes at every moment, when the pumpswash plate position display mode is selected by actuation of thedisplay change-over switch 68. That is, by selecting pump swash plateposition display mode, the motion of the pump swash plate 16A can bechecked easily.

Incidentally, FIG. 6(A) shows the situation where the target actuationposition 69A for accelerating for the pump swash plate 16A is set. FIG.6(B) shows the situation where the pump swash plate 16A is operatedtoward the target actuation position 69A for accelerating. FIG. 6(C)shows the situation where the target actuation position 69A or the limitoperation position 69B for a slowdown of the pump swash plate 16A isset.

When an implement, such as a front loader A, is vertically movablyattached with the height sensor 70 having a potentiometer which detectsthe height position of the implement, the data change means 31C changesthe map data, which correlates the actuation position of the speedchange pedal 24 with the actuation position of the pump swash plate 16Awhich the pump swash plate control means 31A uses, to the map data forimplement lifting stored in the pump swash plate control means 31A basedon detected information from the height sensor 70, as the implement israised to the height position higher than predetermined height (forexample, height position exceeding a car height).

As compared with the map data ordinarily used, the map data forimplement lifting sets the actuation position of the pump swash plate16A for a given actuation position of the speed change pedal 24 to alower speed side. (See FIG. 7). Because the pump swash plate controlmeans 31A uses this map data, the vehicle speed will be restricted tothe low speed side and a high speed travel will be prevented when theimplement is raised higher than a set height.

As shown in FIGS. 4 and 5, the servo control mechanism 25 has a servovalve 27, a regulator valve 28, and the swash-plate sensor 30, and theoil temperature sensor 43 which are housed by the casing 71 removablybolt-connected by the right side part of the second casing part 4B in atransmission case 4 and is formed as a unit electronically operatedmechanism 72. This electronically operated mechanism 72 may be easilychanged to a mechanical type by replacing it with a mechanical unit 76of one block type by incorporating the operating shaft 73 operativelyconnected to the speed change pedal 24 through the linkage mechanism(not shown) of a mechanism type as shown in FIGS. 12 and 13, and theservo valve 74 which has a spool which controls flow of the hydraulicfluid to the cylinder 26 for pumps in the casing 75 which isbolt-connected to the right side part of the second casing part 4B in atransmission case 4.

In addition, the element numbered 77, shown in FIGS. 4 and 12, is a linkarm used in the electronics type servo control mechanism 25 as afeedback arm provided between the cylinder 26 for pumps, and theswash-plate sensor 30, and used, in the mechanical servo controlmechanism 78, as an actuation/feedback combination arm provided betweenthe cylinder 26 for pumps, and the operating shaft 73 to actuate theservo valve 74.

Moreover, the elements numbered 78 and 79 shown in FIGS. 5 and 13 areconnecting openings formed in the abutting surface against the secondcasing part 4B of the casing 71 for connecting to the connectingopenings 80 and 81 when the electronics type operated mechanism 72 isbolt-connected to the right side part of the second casing part 4B.

As shown in FIGS. 4 and 5, the switching mechanism 54 with thechangeover valve 56, the control valve 57, and the high pressureselection valve 58 is formed as a one block unit of the controlmechanism 84 by virtue of being housed by the casing 83 removablybolt-connected to the left side part of the second casing part 4B of thetransmission case 4. The device can be changed from an adjustable motorspecification to a fixed motor specification relatively simply byreplacing the operating mechanism 84 with a plate 85 as shown in FIGS.12 and 14 to cover connecting holes 86-91 formed in the surface with theoperating mechanism 84 of the second casing part 4B and by replacing thevariable capacity motor 17 in the second casing part 4B with the fixedcapacity motor 92, and by removing the cylinder 55 for the motor.

In addition, the elements with reference numbers 93-98 shown in FIG. 5are communicating ports formed in the bonded surface with the secondcasing part 4B of casing 83 that are connected to each correspondingcommunicating ports 86-91 of the second casing part 4B when theswitching mechanism 54 is bolt-connected to the left side part of thesecond casing part 4B.

Because of the above-mentioned structure, the arrangement can simply bechanged among one having the adjustable motor and the servo controlmechanism 25 of the electronic type (see FIG. 5), one with an adjustablemotor and the servo control mechanism 78 of the mechanical type (seeFIG. 13), one with a fixed motor specification and the electronic servocontrol mechanism 25 (see FIG. 14), and one with a fixed motor and theservo control mechanism 78 of the mechanical type (see FIG. 15) whichallows cost reduction because parts may be shared and facilitates partsmanagement.

As shown in FIG. 6, the end regions of the operation area of the speedchange pedal 24 may be set as high-speed regions, and a region betweenthe two ends as a low-speed region, and the motor swash-plate controlmeans 31F may be arranged to control actuation of the control valve 57based on detected information from the pedal sensor 29 such that whenthe speed change pedal 24 is operated to a low-speed region, the motorswash plate 17A switches to a low-speed position, and when the speedchange pedal 24 is operated to a high-speed region, the motor swashplate 17A is located in a high-speed position to use the speed changepedal 24 also as an operating element for a high-low 2 positionchange-over of the variable capacity motor 17. The end regions of theoperation area of the speed change pedal 24 may be set as a high-speedregion, and the motor swash-plate control means 31F may be arranged tocontrol actuation of the control valve 57 based on detected informationfrom the pedal sensor 29 such that when the speed change pedal 24 isoperated to a high-speed region, the motor swash plate 17A is switchedto a high-speed position to use the speed change pedal 24 also as anoperating element for a low-to-high switch of the variable capacitymotor 17.

Thus, when the speed change pedal 24 is used also as an operatingelement for switching the variable capacity motor 17, a detent mechanism(not shown) may be provided for indicating the boundary of the operationarea of the speed change pedal 24.

Another embodiment of a present invention is described next.

The same reference numbers are used for the same parts as theabove-mentioned embodiment, and descriptions of the same parts are notrepeated.

A cruise speed control (a constant speed control) is described next.

As shown in FIGS. 1 and 17, the operator's station 8 has the cruisespeed lever (cruise speed operating tool) 143 only for the forwardtravel speed change which can be held at an arbitrary shift position bya friction type holding mechanism (not shown). As shown in FIG. 17, theposition of the cruise speed lever 143 is detected by the lever sensor(an example of a holding position detection means) 144 which has apotentiometer. A lever sensor 144 outputs the holding position of thedetected cruise speed lever 143 to the control device 31. As shown inFIG. 17, the control means 31 has cruise speed control means 31P forchanging speed in accordance with the actuation position of the cruisespeed lever 143.

As shown in FIGS. 17 and 19, the cruise speed control means 31P has amap data which correlates the holding position of the cruise speed lever143 with the shift operation position of the pump swash plate 16A, and acontrol program which controls actuation of the proportional valve 36for forward travel based on the map data and detected information from alever sensor 144 etc. The map data of the cruise speed control means 31Pcorrelates the actuation position of the cruise speed lever 143 with theshift operation position of the pump swash plate 16A (refer to FIG. 19)such that the greater the operated amount of the cruise speed lever 143from the neutral position (zero speed position) in the forward speedincrease direction is, the greater the operation amount of the pumpswash plate 16A from the neutral position in the direction of forwardtravel is.

The above-mentioned map data may be replaced with a correlation equationthat correlates the actuation position of the cruise speed lever 14 withthe shift operation position of the pump swash plate 16A. The controlprogram of the cruise speed control means 31P sets the shift operationposition of the pump swash plate 16A corresponding to the holdingposition of the cruise speed lever 143 which the lever sensor 144detected as the target actuation position of the pump swash plate 16Abased on the stored map data and detected information from a leversensor 144, and controls the actuation of the proportional valve 36 forforward travel based on the set target actuation position and detectedinformation from the swash-plate sensor 30, so that the target actuationposition of the pump swash plate 16A come into agreement with the actualshift operation position.

This allows a control where the vehicle's forward speed is set accordingto the holding position of the cruise speed lever 143 simply byoperating the cruise speed lever 143 to a desired position against thecomparatively small holding force that holds the cruise speed lever 143in arbitrary actuation positions.

As shown in FIG. 17, the control means 31 has a third operation speedsetting means 31J for setting the operation speed during a speed changeto the speed according to the holding position of the cruise speed lever143. As shown in FIGS. 17 and 18, the third operation speed settingmeans 31J has an operation program which computes the deviation of thetarget actuation position of the pump swash plate 16A, and a actualshift operation position based on the target actuation position of thepump swash plate 16A set by the cruise speed control means 31P, anddetected information from the swash-plate sensor 30, a plurality of mapdata as correlation data which correlates the deviation of the targetactuation position of the pump swash plate 16A and the actual shiftoperation position with the operation speed of the pump swash plate 16A,and a control program which sets the target operation speed of the pumpswash plate 16A based on those map data and calculation results of anoperation program.

Each map data of the third operation speed setting means 31J correlatesthe deviation of the pump swash plate 16A with the operation speed ofthe pump swash plate 16A such that when the deviation of the actualshift operation position of the pump swash plate 16A detected by theswash-plate sensor 30 and the target actuation position of the pumpswash plate 16A set by the cruise speed control means 31P is large, theoperation speed of the pump swash plate 16A is large, and such that theoperation speed of the pump swash plate 16A by control actuation of thecruise speed control means 31P becomes slower than the operation speedof the pump swash plate 16A by control actuation of the speed changecontrol means 31A (dashed line in FIG. 18).

The above-mentioned map data may be replaced by the correlation equationas correlation data that correlates the deviation of the targetactuation position of the pump swash plate 16A and the actual shiftoperation position, with the operation speed of the pump swash plate16A.

The control program of the third operation speed setting means 31J setsthe operation speed of the pump swash plate 16A corresponding to thedeviation of the computed pump swash plate 16A as the target operationspeed of the pump swash plate 16A, based on the stored map data and thecalculation result of an operation program, and outputs the set targetoperation speed to the cruise speed control means 31P.

The control program of the cruise speed control means 31P is arranged tocontrol actuation of the proportional valve 36 for forward travel sothat the pump swash plate 16A is operated at the target operation speedset by the third operation speed setting means 31J. With this control,the operation speed during shift operation of the pump swash plate 16Aby actuation of the cruise speed lever 143 becomes slower than theoperation speed when operating the pump swash plate 16A by actuation ofthe speed change pedal 24. As a result, while improving response inshift operation by the speed change pedal 24, rapid change in thevehicle speed by actuation of the cruise speed lever 143 can beprevented thus making cruise speed setting operation by the cruise speedlever 143 easy to perform.

The data change means 31C changes the map data which the third operationspeed setting means 31J uses, based on the actuation position of theadjustment dial plate 42, into the map data that correlates thedeviation of the pump swash plate 16A with the operation speed of thepump swash plate 16A so that the greater the operation amount of theadjustment dial 42 to the quick side from a reference position is, thespeedier shift operation of the pump swash plate 16A becomes so that theoperation speed of the pump swash plate 16A for a given deviation of thepump swash plate 16A becomes large. And it changes the map data to onethat correlates the deviation of the pump swash plate 16A with theoperation speed of the pump swash plate 16A so that the greater theoperation amount of the adjustment dial 42 is toward the slow side froma reference position, the slower the shift operation of the pump swashplate 16A becomes, and so that the operation speed of the pump swashplate 16A for a given deviation of the pump swash plate 16A becomesslower.

That is, by operating the adjustment dial 42, the actuation feeling of ashifting operation of the hydrostatic equation continuously variablespeed change device 10 with the cruise speed lever 143 can be changedaccording to the liking of an operator as well as the actuation feelingof shift operation of the hydrostatic equation continuously variablespeed change device 10 by the speed change pedal 24, thus improving theactuation feeling or shift operation response.

FIG. 23 shows a curve for the angular position of the pump swash plate16A with respect to time when the adjustment dial 142 is set to thequick side, and a curve for the angular position of the pump swash plate16A with respect to time during cruise speed control and, a curveshowing the angular position of the pump swash plate 16A with respect totime when the adjustment dial 142 set to the slow side.

As shown in FIG. 17, the control device 31 has, as control programs,actuation change-over means 31K which switches the control means tooperate, notifying device change-over means 31G which switches theoperating state of the notifying device 145 which has a lamp in theoperator's station 8, and cruise speed control stop means 31M to stopcontrol actuation of the cruise speed control means 31P.

The notifying device 145 may be a liquid-crystal-display device, abuzzer, etc.

The actuation change-over means 31K is configured to perform thefollowing control. It compare the target actuation position (shiftoperation position of the pump swash plate 16A corresponding to theactuation position of the speed change pedal 24) of the pump swash plate16A set by control of the speed change control means 31A with he targetactuation position (shift operation position of the pump swash plate 16Acorresponding to the holding position of the cruise speed lever 143) ofthe pump swash plate 16A set by control actuation of the cruise speedcontrol means 31P.

The control means is changed based on the comparison so that the controlmeans (between speed change control means 31A and the cruise speedcontrol means 31P) for which the speed corresponding to the targetactuation position is greater is selected to operate.

The notifying device change-over means 31L performs the followingcontrol. The notifying device 145 is switched off based on detectedinformation from the pedal sensor 29, or detected information from alever sensor 144, etc., when the cruise speed lever 143 is detected tobe located in the neutral position, and when a step in operation of thespeed change pedal 24 is detected with the cruise speed lever 143 beingdetected to be located in the neutral position. The notifying device 145is turned on based on detected information from the pedal sensor 29,detected information from a lever sensor 144, etc., when the shiftoperation position of the pump swash plate 16A corresponding to theholding position of the cruise speed lever 143 is detected to be on theacceleration side than the shift operation position of the pump swashplate 16A corresponding to the actuation position of the speed changepedal 24. The notifying device 145 is made to blink (turned on and offin succession), based on detected information from the pedal sensor 29,detected information from a lever sensor 144, etc., while the cruisespeed lever 143 is detected to be located in the actuation position awayfrom the neutral position, and the shift operation position of the pumpswash plate 16A corresponding to the actuation position of the speedchange pedal 24 is detected to be at the shift operation position of thepump swash plate 16A corresponding to the holding position of the cruisespeed lever 143 or on the accelerating side with respect to the shiftoperation position.

The cruise speed control stop means 31M is configured to perform thefollowing control. When the step in operation to the reverse travelspeed change region of the speed change pedal 24 is detected based ondetected information from the pedal sensor 29 during the cruise speedforward operation under control of the cruise speed control means 31P,or when both of the pair of side brake pedals 147 in the operator'sstation 8 are detected to be operated on based on detected informationfrom the brake sensor 146, actuation of the proportional valve 36 forforward travel is controlled so that the pump swash plate 16A isoperated toward the neutral position at the operation speed for apredetermined cruise speed control stop.

At this point in time, unless the actuation to the neutral position ofthe cruise speed lever 143 is detected based on detected informationfrom a lever sensor 144, control actuation of the cruise speed controlmeans 31P is stopped.

Moreover, when it is detected based on detected information from a leversensor 144 at the time of start up of an engine 1 that the cruise speedlever 143 is not located in the neutral position, or when acceleratingactuation of the cruise speed lever 143 is detected based on detectedinformation from a lever sensor 144 when traveling in reverse, controlactuation of the cruise speed control means 31P is stopped until itdetects the actuation of the cruise speed lever 143 to the neutralposition. If the speed change pedal 24 is operated forwardly in theforward travel direction while the cruise speed lever 143 is located inthe neutral position, the pump swash plate 16A is operated to a positionto a forward travel accelerating side corresponding to the actuationposition of the speed change pedal 24, and the vehicle body movesforward at the speed in accordance with the shift operation position.When this happens, the notifying device 145 is turned off to notify thatthe system is in the normal traveling condition where shift operationonly by the speed change control means 31A is performed.

If the speed change pedal 24 is operated to the reverse travel sidewhile the cruise speed lever 143 is located in the neutral position, thepump swash plate 16A is operated to the shift operation position to theside of reverse travel corresponding to the actuation position of thespeed change pedal 24, and the vehicle body travels in reverse at thespeed dictated by the shift operation position.

The notifying device 145 is turned off also in this case to notify thatthe system is in the normal traveling condition. If a pivoting operationis pivoted in the accelerating direction of the cruise speed lever 143with the speed change pedal 24 located in the neutral position, the pumpswash plate 16A will be shifted to the shift operation position to theforward accelerating travel side corresponding to the holding positionof the cruise speed lever 143, and a vehicle body will carry out acruise speed forward travel at the speed set by the shift operationposition. In this case, the notifying device 145 is turned on to notifythat the system is in the cruise speed forward condition under controlof the cruise speed control means 31P.

If the cruise speed lever 143 is pivoted to the accelerating directionduring a forward travel by actuation of the speed change pedal 24, thevehicle body moves forward at a speed corresponding to the actuationposition of the speed change pedal 24 until the target actuationposition (target actuation position of the cruise speed control means31P) of the pump swash plate 16A set by control of the cruise speedcontrol means 31P is on the accelerating side with respect to the targetactuation position (target actuation position in the speed changecontrol means 31A) of the pump swash plate 16A set by control of thespeed change control means 31A. During this time, the notifying device145 goes out to notify that the system is in a normal travelingcondition. At this point in time, if the target actuation position inthe cruise speed control means 31P is on an accelerating side ratherthan the target actuation position in the speed change control means 31Aby a pivoting operation -to the accelerating direction of the cruisespeed lever 143, or the deceleration operation of the speed change pedal24, the vehicle body moves forward at the speed according to theactuation position of the cruise speed lever 143, resulting in thecruise speed travel according to the holding position of the cruisespeed lever 143.

In this case, the notifying device 145 is turned on to notify that thesystem is in the cruise speed forward condition.

If the speed change pedal 24 is operated to the forward acceleratingtravel direction during the cruise speed forward condition based onactuation of the cruise speed lever 143, the vehicle carries out cruisespeed traveling at the speed according to the holding position of thecruise speed lever 143 until the target actuation position set by thespeed change control means 31A is the same as the target actuationposition set by the cruise speed control means 31P, or is on anaccelerating side with respect to the target actuation position set bythe cruise speed control means 31P.

During this time, the notifying device 145 is turned on to notify thatthe system is in the cruise speed forward condition. At this time, thevehicle moves forward at the speed according to the actuation positionof the speed change pedal 24 if the target actuation position set by thespeed change control means 31A becomes the same as the target actuationposition set by the cruise speed control means 31P, or is on anaccelerating side with respect to the target actuation position set bythe cruise speed control means 31P by a operation of the speed changepedal 24 to the direction of forward accelerating travel.

In this case, the notifying device 145 blinks to notify that the systemis in the accelerating precedence condition where priority is given tothe shift operation by control of the speed change control means 31A inthe cruise speed forward condition by control of the cruise speedcontrol means 31H. At this time, if the target actuation position in thespeed change control means 31A comes to be on a slowdown side withrespect to the target actuation position in the cruise speed controlmeans 31P by the deceleration operation of the speed change pedal 24,the vehicle will carry out cruise speed traveling at the speed accordingto the holding position of the cruise speed lever 143 again.

In this case, the notifying device 145 lights up to notify that thesystem is in the cruise speed forward condition.

If the speed change pedal 24 is operated to the direction of reverseaccelerating travel or both of the side brake pedals 147 are actuatedduring the cruise speed forward condition based on actuation of thecruise speed lever 143, the vehicle speed is gradually slowed downtoward the speed set by the actuation position of the speed change pedal24 from the speed set by the holding position of the cruise speed lever143, and the vehicle runs at the speed according to the actuationposition of the speed change pedal 24. In this case, the notifyingdevice 145 is turned off to notify that the system is in the normaltraveling condition.

After the speed change pedal 24 is operated to the direction of reverseaccelerating travel or after both of the side brake pedals 147 areactuated during the cruise speed forward condition based on actuation ofthe cruise speed lever 143, it becomes possible to carry out the cruisespeed forward travel of the vehicle at the speed according to theactuation position of the cruise speed lever 143 by returning the cruisespeed lever 143 to the neutral position.

The speed maintenance mechanism A is has the cylinder 26 for pumps, aregulator valve 28, the swash-plate sensor 30, the control device 31,the proportional valve 36 for forward travel, the cruise speed lever143, the lever sensor 144, etc.

The above-mentioned structure enables advantageous coordination betweenthe shift operation by the speed change pedal 24, and the shiftoperation by the cruise speed lever 143 while the operator is made awareof the traveling condition initiated by these operation.

A neutral return mechanism may be provided which, for example, may havean electromagnetic cylinder etc., so that the cruise speed lever 143 isautomatically returned to the neutral position, when the speed changepedal 24 is operated to the direction of reverse accelerating travel orafter both of the side brake pedals 147 are actuated during the cruisespeed forward condition based on actuation of the cruise speed lever143, so as to improve the shift operation further.

As shown in FIGS. 20 and 21, the brake sensor 146 has a single normallyclosed switch 148 and a pair of bent links 149. The bent links 149 arearranged between the coordinated arm 147A of the corresponding sidebrake pedal 147, and the panel frame 50 located forwardly of the sidebrake pedal 147 such that they can expand and contract.

The normally closed switch 148 is held in the open condition by pressingoperation by both bent links 149 in the non-braking state where bothside brake pedals 147 are not actuated [see FIG. 20]. During the brakingturning state where one of the side brake pedals 147 is pressed down,the circuit is kept open by the pressing operation by the bent link 149connected to the side brake pedal 147 which is not actuated [see FIG.21(A)].

In the braking state where both side brake pedals 147 are actuated, thecircuit returns to a closed state due to the fact that neither of thetwo bent links 149 are pressing down on the switch [see FIG. 21(B)].

That is, a simple structure having the single normally closed switch 148for a pair of side brake pedals 147, can ensure reliable detectedinformation from actuation of both brake pedals 147.

When shift operation of the pump swash plate 16A is performed by theelectronic type servo control mechanism 25 as shown in FIG. 24, ahysteresis will occur between the actuation position of the pump swashplate 16A, and a servo pressure (operating physical force to the pumpswash plate 16A). Therefore, when shift operation of the pump swashplate 16A is performed by control actuation of the speed change controlmeans 31A based on actuation of the speed change pedal 24 without takingthis hysteresis into consideration, when switching between theacceleration operation and the decelerating by the speed change pedal 24is performed, the difference of the servo pressure at the time of theaccelerating actuation to the current position of the pump swash plate16A, and the servo pressure at the time of deceleration operation by thehysteresis causes the pump swash plate 16A to be maintained despite anactuation of the speed change pedal 24 in a current position until thedifference is canceled by actuation of the servo control mechanism 25based on actuation of the speed change pedal 24.

That is, in the shift operation after an by the speed change pedal 24,the response of the pump swash plate 16A will fall, and the operator mayfeel the adverse effect. To this end, as shown in FIG. 17, the controldevice 31 has the compensation means 31Q which compensates for thedifference between the servo pressure at the time of the acceleratingactuation to the current position of the pump swash plate 16A and theservo pressure at the time of deceleration operation when theacceleration or deceleration operation by the speed change pedal 24.

The compensation means 31Q has a operation program which computes theactuation position after the fixed time period of the speed change pedal24 based on detected information from the pedal sensor 29, correlationdata which correlates the servo pressure at the time of the acceleratingactuation for a given actuation position of the pump swash plate 16Awith the servo pressure at the time of deceleration operation, and acontrol program which controls actuation of a servo valve 27 based onthe calculation result of the operation program and correlation data.

Based on detected information from the pedal sensor 29, the operationprogram of the compensation means 31Q detects the current position θ ofthe speed change pedal 24 and computes operation speed ω and predictsthe actuation position β(=θ+ωt) of the speed change pedal 24 after afixed time from those detection results and calculation results. As thecorrelation data of the compensation means 31Q has a value (averagevalue) Δi of the difference of the current value Ia supplied to a servovalve 27 at the time of accelerating actuation, and the current value Ibsupplied to a servo valve 27 at the time of deceleration operation for agiven actuation position of the pump swash plate 16A, which is requiredto compensate for the difference Δf between the servo pressure Fa at thetime of the accelerating actuation and the servo pressure Fb at the timeof deceleration operation for a given actuation position of the pumpswash plate 16A.

The control program of the compensation means 31Q compares the predictedactuation position β, which is the calculation result of the operationprogram, with the current position θ, and controls actuation of theservo valve 27 so that the difference Δf of the servo pressure Fa at thetime of the accelerating actuation and the servo pressure Fb at the timeof deceleration operation for the current position of the pump swashplate 16A is compensated based on the correlation data, when thedifference becomes beyond the predetermined setting α.

More specifically, when the switch to forward travel decelerationoperation from forward travel accelerating actuation of the speed changepedal 24 is detected based on detected information from the pedal sensor29, the current position θ of the obtained speed change pedal 24 iscompared with the predicted actuation position β, which is a calculationresult of the operation program. When the difference becomes greaterthan the predetermined value (β<θ−α), in order to decrease the servopressure F corresponding to the current position of the pump swash plate16A promptly from the servo pressure Fa at the time of acceleratingactuation to the servo pressure Fb at the time of decelerationoperation, the current is lowered at once to the current value Ib forobtaining the servo pressure Fb at the time of the decelerationoperation corresponding to the current position of the pump swash plate16A by subtracting the difference value Δi from the current value Ia forobtaining the servo pressure Fa at the time of the acceleratingactuation corresponding to the current position of the pump swash plate16A.

When the switch to the forward travel accelerating actuation from theforward travel deceleration operation of the speed change pedal 24 isdetected based on detected information from the pedal sensor 29, thecurrent position θ of the speed change pedal 24 obtained at that time iscompared with the predicted actuation position β, which is thecalculation result of an operation program. When the difference becomesgreater than the predetermined value (β>θ−α) in order to increase theservo pressure F corresponding to the current position of the pump swashplate 16A promptly from the servo pressure Fb at the time ofdeceleration operation to the servo pressure Fa at the time ofaccelerating actuation, the current is raised at once to the currentvalue Ia for obtaining the servo pressure Fa at the time of theaccelerating actuation corresponding to the current position of the pumpswash plate 16A by adding the difference value Δi to the current valueIb for obtaining the servo pressure Fb at the time of the decelerationoperation corresponding to the current position of the pump swash plate16A. When a switch to reverse travel deceleration operation from reversetravel accelerating actuation of the speed change pedal 24 is detectedbased on detected information from the pedal sensor 29, the currentposition θ of the speed change pedal 24 obtained at that time iscompared with the predicted actuation position β, which is a calculationresult of an operation program. When the difference becomes greater thanthe predetermined setting (β<θ−α), in order to lower the servo pressureF corresponding to the current position of the pump swash plate 16Apromptly from the servo pressure Fa at the time of acceleratingactuation to the servo pressure Fb at the time of decelerationoperation, the current is lowered at once to the current value Ib forobtaining the servo pressure Fb at the time of the decelerationoperation corresponding to the current position of the pump swash plate16A by subtracting the difference value Δi from the current value Ia forobtaining the servo pressure Fa at the time of the acceleratingactuation corresponding to the current position of the pump swash plate16A.

When switch to the reverse travel accelerating actuation from thereverse travel deceleration operation of the speed change pedal 24 isdetected based on detected information from the pedal sensor 29, thecurrent position θ of the speed change pedal 24 then obtained iscompared with the predicted actuation position β, which is thecalculation result of the operation program. When the difference becomesgreater than the predetermined setting (β>θ−α), in order to increase theservo pressure F corresponding to the current position of the pump swashplate 16A promptly from the servo pressure Fb at the time ofdeceleration operation to the servo pressure Fa at the time ofaccelerating actuation, the current is increased at once to the currentvalue Ia for obtaining the servo pressure Fa at the time of theaccelerating actuation corresponding to the current position of the pumpswash plate 16A by adding the difference value Δi to the current valueIb for obtaining the servo pressure Fb at the time of the decelerationoperation corresponding to the current position of the pump swash plate16A.

Therefore, in the forward travel deceleration operation after switchingto the forward travel slowdown from forward travel accelerating of thespeed change pedal 24, the servo pressure Fa corresponding to the pumpswash plate 16A falls to the servo pressure Fb at the time of theforward travel deceleration operation corresponding to the actuationposition of the speed change pedal 24 promptly with actuation of thespeed change pedal 24, which improves response of a decelerationoperation of the pump swash plate 16A to the shift operation positionfor forward travel corresponding to the actuation position of the speedchange pedal 24.

In the forward travel accelerating actuation after switching to forwardtravel accelerating from the forward travel slowdown of the speed changepedal 24, the servo pressure Fb for the pump swash plate 16A raisespromptly with actuation of the speed change pedal 24 to the servopressure Fa at the time of the forward travel accelerating actuationcorresponding to the actuation position of the speed change pedal 24,which improves the response in accelerating actuation of the pump swashplate 16A to the shift operation position for forward travelcorresponding to the actuation position of the speed change pedal 24.

In the reverse travel deceleration operation after switching to thereverse travel decelerating from reverse travel accelerating of thespeed change pedal 24, the servo pressure Fa to the pump swash plate 16Afalls promptly to the servo pressure Fb at the time of the reversetravel deceleration operation corresponding to the actuation position ofthe speed change pedal 24 with actuation of the speed change pedal 24,which improves response in a deceleration operation of the pump swashplate 16A to the shift operation position for reverse travelcorresponding to the actuation position of the speed change pedal 24 bythe response.

In the reverse travel accelerating actuation after switching to reversetravel acceleration from the reverse travel deceleration of the speedchange pedal 24, the servo pressure Fb for the pump swash plate 16Arises promptly with actuation of the speed change pedal 24 to the servopressure Fa at the time of the reverse travel accelerating actuationcorresponding to the actuation position of the speed change pedal 24,which improves response in accelerating actuation of the pump swashplate 16A to the shift operation position for reverse travelcorresponding to the actuation position of the speed change pedal 24.

That is, because of the ability to perform shift operation of the pumpswash plate 16A in consideration of the hysteresis which exists betweenthe actuation position of the pump swash plate 16A, and a servopressure, by the electronic type servo control mechanism 25, theresponse delay of the pump swash plate 16A resulting from the hysteresiscan be controlled effectively, improving response quality to operatoractuation. The hysteresis which exists between the actuation position ofthe pump swash plate 16A and a servo pressure is influenced by thetemperature of the hydraulic fluid supplied to the cylinder 26 forpumps.

The lower the temperature of the hydraulic fluid is, the greater thedifference Δf of the servo pressure Fa at the time of the acceleratingactuation to the actuation position of the pump swash plate 16A and theservo pressure Fb at the time of deceleration operation becomes.Accordingly, the hydraulic circuit to the cylinder 26 for pumps has theoil temperature sensor 43 which detects the temperature of the hydraulicfluid supplied to a regulator valve 28. And the compensation means 31Qtakes into consideration of the fact that the lower the temperature ofthe hydraulic fluid supplied to a regulator valve 28 is, the greater thedifference Δf of the servo pressure Fa at the time of the acceleratingactuation to the actuation position of the pump swash plate 16A and theservo pressure Fb at the time of deceleration operation becomes. And themeans 31Q is configured to change the difference value Δi between thecurrent value Ia supplied to a servo valve 27 at the time ofaccelerating actuation and the current value Ib supplied to a servovalve 27 at the time of deceleration operation based on detectedinformation from the oil temperature sensor 43.

To describe in more detail, the compensation means 31Q has the map datawhich correlates the temperature of the hydraulic fluid supplied to aregulator valve 28 with the correction coefficient which compensates thecurrent difference value Δi. Map data correlates the temperature withthe correction coefficient of hydraulic fluid such that the lower thetemperature of hydraulic fluid is, the greater the value Δi of thedifference of a current value becomes. And the control program of thecompensation means 31Q selects the correction coefficient according tothe temperature of the hydraulic fluid based on detected informationfrom the map data and oil temperature sensor 43, and multiply the valueΔi of the difference of a current value by the correction coefficient,so that the value Δi of the difference of a current value is amended tothe proper value according to the temperature of the hydraulic fluid atthat time.

That is, in response to the fact that, the lower the temperature of thehydraulic fluid supplied to the regulator valve 28 is, the greater thedifference Δf of the servo pressure Fa at the time of the acceleratingactuation to the actuation position of the pump swash plate 16A and theservo pressure Fb at the time of deceleration operation becomes. And thecurrent difference value Δi is changed correspondingly to a largervalue.

Thus, when acceleration or deceleration operation by the speed changepedal 24 is detected, the servo pressure F corresponding to the currentposition of the pump swash plate 16A can be promptly changed into theservo pressure corresponding to the shift operation after switchactuation regardless of the temperature of the hydraulic fluid suppliedto the cylinder 26 for pumps. As a result, this allows shift operationwhich can control the response delay in consideration of change of thehysteresis by the temperature of the hydraulic fluid supplied to thecylinder 26 for pumps of the pump swash plate 16A.

The control device 31 has, as a control program, the commanding meanswhich orders a start and a stop of control actuation to the speed changecontrol means 31A.

The commanding means issues a command to stop the control to the speedchange control means 31A when the speed change pedal 24 is detected tobe operated in the neutral position based on detected information fromthe pedal sensor 29, if the pump swash plate 16A is detected to havearrived at the setting position near the predetermined neutral positionbased on detected information from the swash-plate sensor 30, and if thevehicle speed is detected to have fallen to the low set speed set inadvance, based on the output of the vehicle speed sensor (speeddetecting means) 50 which detects the vehicle speed from the outputrotation rate of the gear type speed change device 11.

Also, the speed change control means 31A is ordered to stop the controlwhen the speed change pedal 24 is not detected to have been operated tothe neutral position based on detected information from the pedal sensor29, and if the pump swash plate 16A is detected to have been operated tothe shift operation region opposite beyond the neutral position, basedon detected information from the swash-plate sensor 30.

That is, when the speed change pedal 24 arrives at the neutral positionby the deceleration operation of the speed change pedal 24, the pumpswash plate 16A is operated toward the shift operation position (neutralposition) corresponding to the neutral position of the speed changepedal 24 by control actuation of the speed change control means 31Abased on the actuation, and by urging force of the forward traveldecelerating spring 32 or the reverse travel decelerating spring 33.

When the pump swash plate 16A arrives at the setting position near thepredetermined neutral position by this decelerating operation, it isdetermined whether the vehicle speed fell to the predetermined setspeed. When it has not fell to a set speed, control actuation of thespeed change control means 31A is continued.

On the other hand, if the vehicle speed falls to predetermined setspeed, the speed change control means 31A will discontinue controlactuation, and the pump swash plate 16A moves to the neutral position bythe urging force of the forward travel decelerating spring 32 or thereverse travel decelerating spring 33.

Moreover, up to the time the speed change pedal 24 arrives at theneutral position by the deceleration operation of the speed change pedal24 or when it reaches the neutral position, if the pump swash plate 16Ais operated to the shift operation region to the opposite side beyondthe neutral position, the speed change control means 31A suspends thecontrol operation, so that the pump swash plate 16A moves to the neutralposition by the urging force of the forward travel decelerating spring32 or the reverse travel decelerating spring 33.

Various setting is possible for the set speed. Here, it is set to a lowenough speed so that the pump swash plate 16A would not be operated inthe accelerating direction by the running inertia against the urging ofthe forward travel decelerating spring 32 or the reverse traveldecelerating spring 33. As a result, despite the difference between thepump swash plate 16A and the swash-plate sensor 30 caused by the passageof time, when the speed change pedal 24 is located in the neutralposition, the pump swash plate 16A would reliably be located in theneutral position.

This prevents a problem of the vehicle running despite the speed changepedal 24 being in the neutral position because of the discrepancybetween the pump swash plate 16A and the swash-plate sensor 30. And inthis condition, since the vehicle speed is sufficiently low,accelerating actuation of the pump swash plate 16A caused by the inertiaduring travel is avoided.

When the pump swash plate 16A is located in the neutral position by theurging force of the forward travel decelerating spring 32 or the reversetravel decelerating spring 33, the commanding means compares the realneutral position (real zero speed position) of the pump swash plate 16Adetected by the swash-plate sensor 30 with the set neutral position (setzero speed position) of the pump swash plate 16A which was set tocorrespond to the neutral position of the speed change pedal. When it isdetected that the speed change pedal 24 is operated in the directioncorresponding to the direction in which the real neutral position of thepump swash plate 16A is displaced with respect to the set neutralposition of the pump swash plate 16A, based on detected information fromthe comparison result and pedal sensor 29, the command means orderssuspension of control by the speed change control means 31A until thespeed change pedal 24 arrives at the actuation position corresponding tothe real neutral position of the pump swash plate 16A.

And when the speed change pedal 24 arrives at a corresponding actuationposition to the real neutral position of the pump swash plate 16A, thespeed change control means 31A is ordered to start control. On the otherhand, if it is detected that the speed change pedal 24 is operated inthe direction opposite to the direction corresponding to the directionin which the real neutral position of the pump swash plate 16A isdisplaced with respect to the set neutral position of the pump swashplate 16A, the speed change control means 31A is ordered to initiatecontrol. Thus, when accelerating actuation of the speed change pedal 24is carried out in the direction corresponding to the direction in whichthe real zero speed position of the pump swash plate 16A is displacedwith respect to the set zero speed position of the pump swash plate 16A,the pump swash plate 16A moves to the neutral position by the urgingforce of the forward travel decelerating spring 32 and the reversetravel decelerating spring 33 until the speed change pedal 24 arrives atthe actuation position corresponding to the real zero speed position ofthe pump swash plate 16A.

And as the speed change pedal 24 arrives at the actuation positioncorresponding to the real zero speed position of the pump swash plate16A, the speed change control means 31A starts control.

Accelerating actuation of the pump swash plate 16A according toactuation of the speed change pedal 24 is carried out by the controlagainst the urging force of the forward travel decelerating spring 32 orthe reverse travel decelerating spring 33. On the other hand, whenaccelerating actuation of the speed change pedal 24 is carried out inthe direction opposite to the direction corresponding to the directionin which the real zero speed position of the pump swash plate 16A isdisplaced with respect to the setting zero speed position of the pumpswash plate 16A, the speed change control means 31A starts control, andan accelerating actuation of the pump swash plate 16A according to theactuation of the speed change pedal 24 is carried out against the urgingforce of the forward travel decelerating spring 32 or the reverse traveldecelerating spring 33.

As a result, this solves the problem that, during accelerating actuationfrom the zero speed position of the speed change pedal 24, the pumpswash plate 16A is operated in the direction opposite to the directioncorresponding to the manipulating direction of the speed change pedal 24due to discrepancies between the pump swash plate 16A and theswash-plate sensor 30 caused by passage of time, resulting in thevehicle running in the direction opposite to the manipulating directionof the speed change pedal 24.

Other Embodiments

[1] The work vehicle may be a riding type rice planting machine, ariding type mowing machine, or a wheel loader.

[2] The continuously variable speed change device 10 may be a belt typeetc.

[3] Correlation data may make have a plurality of correlating equationswhich corresponded to various conditions and/or may have multiplierscorresponding to various conditions.

[4] Planetary gear transmission may be used instead of the stepwisechange speed device 11.

[5] An electric cylinder, an electric motor or a hydraulic motor, etc.may be used as a control means 55.

[6] The speed change control 24 may be a speed change lever etc.

1. A load control structure for a work vehicle comprising: set rotationspeed detection means for detecting a set rotation of an engine of thework vehicle; actual rotation speed detection means that senses anactual rotational speed of the engine; continuously variable speedchange device that receives power from the engine of the work vehicle;speed change position detecting means for detecting a speed changeoperation position of the continuously variable speed change device;operating means for speed-shifting the continuously variable speedchange device; control means for controlling the operation of theoperating means; operation speed setting means for setting an operationspeed at which the continuously variable speed change device isoperated; wherein the control means calculates a drop amount of theactual engine rotation speed from the set rotation speed based on thedetected information from the set rotation speed detection means and theactual rotation speed detection means, and sets a limit operationposition for the continuously variable speed change device based on thecalculated drop amount and a correlation data that correlates the actualengine rotation speeds with operating positions of the continuouslyvariable speed change device, and controls the operating means such thatthe operating position of the continuously variable speed change devicemoves to the limit operating position based on the set limit operatingposition and detected information from the speed change positiondetecting means, and wherein the control means controls the operation ofthe operating means such that the continuously variable speed changedevice is operated by the operating means at the operation speed set bythe operation speed setting means.
 2. A load control structure for workvehicle according to claim 1, wherein the operation speed setting meanscalculates a rate of change in the actual engine rotation speed based onthe detected information from the rotation speed detection means, andsets the operation speed such that the greater the calculated rate ofchange is, the greater the operation speed is.
 3. A load controlstructure for work vehicle according to claim 1, wherein the structurefurther has a stepwise speed change device and the stepwise speed changeposition detection means for detecting the speed position of thestepwise speed change device, and wherein the operation speed settingmeans controls the operation means, based on the detected informationfrom the stepwise speed change position detection means, such that thelower the speed position of the stepwise speed change device is, thegreater the speed at which the continuously variable speed change deviceis operated toward the limit operation position is.
 4. A load controlstructure for work vehicle according to claim 1, wherein the controlmeans never sets the limit operation position for the continuouslyvariable speed change position to the neutral position.
 5. A method forcontrolling a speed of a work vehicle, the work vehicle having setrotation speed detection means for detecting a set rotation of an engineof the work vehicle; actual rotation speed detection means that sensesan actual rotational speed of the engine; continuously variable speedchange device that receives power from the engine of the work vehicle;speed change position detecting means for detecting a speed changeoperation position of the continuously variable speed change device;operating means for speed-shifting the continuously variable speedchange device; control means for controlling the operation of theoperating means; and operation speed setting means for setting anoperation speed at which the continuously variable speed change deviceis operated the method comprises the steps of: calculating a drop amountof the actual engine rotation speed from the set rotation speed based onthe detected information from the set rotation speed detection means andthe actual rotation speed detection means; setting a limit operationposition for the continuously variable speed change device based on thecalculated drop amount and a correlation data that correlates the actualengine rotation speeds with operating positions of the continuouslyvariable speed change device; controlling the operating means such thatthe operating position of the continuously variable speed change devicemoves to the limit operating position based on the set limit operatingposition and detected information from the speed change positiondetecting means; and controlling the operation of the operating meanssuch that the continuously variable speed change device is operated bythe operating means at the operation speed set by the operation speedsetting means.
 6. A method according to claim 5, further comprising:calculating a rate of change in a drop amount of the actual enginerotation speed based on the detected information from the set rotationspeed detection means; setting a operation speed at which thecontinuously variable change speed device is operated such that thegreater the rate of change is, the greater the operation speed of thecontinuously variable speed change device is.
 7. A method according toclaim 5, wherein the structure further has a stepwise speed changedevice and the stepwise speed change position detection means fordetecting the speed position of the stepwise speed change device, andwherein the method includes controlling the operation means, based onthe detected information from the stepwise speed change positiondetection means, such that the lower the speed position of the stepwisespeed change device is, the greater the speed at which the continuouslyvariable speed change device is operated toward the limit operationposition is.
 8. A method according to claim 5, wherein the control meansnever sets the limit operation position for the continuously variablespeed change position to the neutral position.